Antiferromagnetic storage device

ABSTRACT

An antiferromagnetic nanostructure according to one embodiment includes an array of at least two antiferromagnetically coupled magnetic atoms having at least two magnetic states that are stable for at least one picosecond even in the absence of interaction with an external structure, the array having a net magnetic moment of zero or about zero, wherein the array has 100 atoms or less along a longest dimension thereof. An atomic-scale structure according to one embodiment has a net magnetic moment of zero or about zero; two or more stable magnetic states; and having an array of atoms that has magnetic moments that alternate between adjacent magnetic atoms along one or more directions. Such structures may be used to store data at ultra-high densities.

BACKGROUND

The invention is in the field of the physics of data storage andcomputation, and more particularly, this invention relates to theability to store magnetic information in antiferromagneticnanostructures.

The ability to record digital information in the magnetic orientation ofmagnetic grains is at the heart of data storage in informationtechnology devices. For several decades there has been exponentialprogress in reducing the effective area of individual magnetic elements.This basic idea has been applied to magnetic hard disk media, tomagnetic tape media, and most recently to solid-state implementationssuch as spin-transfer torque magnetic random access memory. At presentall of these devices utilize a ferromagnetic interaction between themagnetic atoms that constitute the active device element that stores theinformation bit. In disk and tape drives the magnetic information isread out by magnetoresistive sensors, which sense the magnetic fieldemanating from the ferromagnetic bit. Writing of the magnetic elementsis achieved by creating a strong localized magnetic field from awrite-head. In solid-state magnetic devices the magnetic bit istypically part of a magnetoresistive tunneling junction, which can beused for reading and writing the information.

BRIEF SUMMARY

An antiferromagnetic nanostructure according to one embodiment includesan array of at least two antiferromagnetically coupled magnetic atomshaving at least two magnetic states that are stable for at least onepicosecond even in the absence of interaction with an externalstructure, the array having a net magnetic moment of zero or about zero,wherein the array has 100 atoms or less along a longest dimensionthereof.

An atomic-scale structure according to one embodiment has a net magneticmoment of zero or about zero; two or more stable magnetic states; andhaving an array of atoms that has magnetic moments that alternatebetween adjacent magnetic atoms along one or more directions.

An antiferromagnetic nanostructure according to one embodiment includesmultiple arrays each corresponding to a bit, each array having at leasteight antiferromagnetically coupled magnetic atoms, each array having atleast two readable magnetic states that are stable for at least onepicosecond, each array having a net magnetic moment of zero or aboutzero, wherein no external stabilizing structure exerts influence overthe arrays for stabilizing the arrays, wherein each array has 100 atomsor less along a longest dimension thereof.

In yet another embodiment, a system, such as a magnetic data storagesystem or memory device, may include an antiferromagnetic nanostructureand/or atomic-scale structure as recited above; and at least one devicefor altering and/or reading the magnetic state of each of the arrays

Other aspects and embodiments of the present invention will becomeapparent from the following detailed description, which, when taken inconjunction with the drawings, illustrate by way of example theprinciples of the invention.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIGS. 1A-1C are graphical depictions of an array ofantiferromagnetically coupled magnetic atoms according to oneembodiment.

FIG. 2 is graphical representation of a scanning tunneling microscope(STM) topographic image of a chain of 8 Fe atoms according to oneembodiment.

FIG. 3 is a graphical depiction of an implementation of reading andwriting operations of a magnetic nanostructure by using the tip of anSTM according to one embodiment.

FIGS. 4A-4B are graphical depictions of an array ofantiferromagnetically coupled magnetic atoms according to oneembodiment.

FIG. 5A is a graphical depiction of an array of antiferromagneticallycoupled magnetic atoms in a first state according to one embodiment.

FIG. 5B is a graphical depiction of changing the state of the array ofFIG. 5A according to one embodiment.

FIG. 5C is a graphical depiction of the array of FIG. 5A in a secondstate according to one embodiment.

FIG. 6 is a graphical depiction of multiple arrays ofantiferromagnetically coupled magnetic atoms on a common substrate,according to one embodiment.

FIG. 7 is a graphical depiction of multiple arrays ofantiferromagnetically coupled magnetic atoms on a common substrate,according to one embodiment.

FIG. 8 is a graphical depiction of multiple arrays ofantiferromagnetically coupled magnetic atoms on a common substrate,according to one embodiment.

FIGS. 9A-9B are graphical depictions of arrays of antiferromagneticallycoupled magnetic atoms extending in three dimensions, according to oneembodiment.

FIG. 10 is a graphical depiction of an implementation of a reading orwriting operation of a magnetic nanostructure by using the tip of an STMaccording to one embodiment.

FIG. 11A is a chart of a tunnel current as a function of time with anSTM tip positioned over an end atom of an array of antiferromagneticallycoupled atoms according to one embodiment.

FIG. 11B is a chart of a tunnel current as a function of time with anSTM tip positioned over an end atom of an array of antiferromagneticallycoupled atoms according to one embodiment.

FIG. 11C is a chart showing switching rates at various tunnel currentlevels according to one embodiment.

FIG. 11D is a chart showing switching rates at various voltage levelsaccording to one embodiment.

FIG. 12A is a graphical depiction of a (2×6) and (2×4) array of Fe atomsaccording to one embodiment.

FIG. 12B is a schematic of atomic positions of Fe and Cu₂N substrateatoms in a (2×n) and (1×n) arrays according to one embodiment.

FIG. 12C is an Arrhenius plot of the residence time for the arrays ofFIG. 12A and the (1×8) array of FIG. 10.

FIG. 13A is a depiction of eight (2×6) arrays of antiferromagneticallycoupled atoms according to one embodiment.

FIG. 13B is a partial graphical depiction of the bits of FIG. 13Aaccording to one embodiment.

FIG. 13C is a partial graphical depiction of the bits of FIG. 13Aaccording to one embodiment.

DETAILED DESCRIPTION

The following description is made for the purpose of illustrating thegeneral principles of the present invention and is not meant to limitthe inventive concepts claimed herein. Further, particular featuresdescribed herein can be used in combination with other describedfeatures in each of the various possible combinations and permutations.

Unless otherwise specifically defined herein, all terms are to be giventheir broadest possible interpretation including meanings implied fromthe specification as well as meanings understood by those skilled in theart and/or as defined in reference books such as dictionaries,treatises.

It must also be noted that, as used in the specification and theappended claims, the singular forms “a,” “an” and “the” include pluralreferents unless otherwise specified.

The following description discloses several preferred embodiments ofantiferromagnetic nanostructures capable of storing information. Alsodisclosed are tunneling magnetoresistance devices for writing andreading the information.

In one general embodiment, an antiferromagnetic nanostructure includesan array of at least two antiferromagnetically coupled magnetic atomshaving at least two magnetic states that are stable for a useful periodof time such as at least one picosecond even in the absence ofinteraction with an external structure such as a ferromagnetic structureexternal to the array for stabilizing the array in at least one of themagnetic states. Thus, an external stabilizing structure is not present,nor required, in preferred embodiments. The array has a net magneticmoment of zero or about zero. The array has 100 atoms or less along alongest dimension thereof, i.e., the maximum number of atoms lying alonga line in any dimension is 100 atoms. The total number of atoms in thearray, then, may be 100 for a single line array, or more for 2D and 3Darrays having more than one atom in the second and/or third dimensions.

By “about zero” when referring to a net magnetic moment, what is meantis that the net magnetic moment per magnetic atom of the subject arrayis less than about 20% of the average magnetic moment of the magneticatoms in the array. In preferred embodiments, the net magnetic momentper magnetic atom of each array is less than about 10% of the averagemagnetic moment of the magnetic atoms, and ideally less than about 5%.

In another general embodiment, an atomic-scale structure has a netmagnetic moment of zero or about zero; two or more stable magneticstates; and includes an array of atoms that have magnetic moments thatalternate between adjacent magnetic atoms along one or more directions.An “atomic-scale structure” may be defined as a structure having atleast two atoms, and having 100 atoms or less along a longest dimensionthereof, i.e., the maximum number of atoms lying along a line in anydimension is 100 atoms.

In yet another general embodiment, an anti ferromagnetic nanostructureincludes multiple arrays each corresponding to a data bit (also referredto herein as a magnetic bit), each array having at least eightantiferromagnetically coupled magnetic atoms, each array having at leasttwo readable magnetic states that are stable for at least one picosecond(or other useful period of time), each array having a net magneticmoment of zero or about zero, where no external stabilizing structuresare required to exert influence over the arrays stabilizing the arrays,and where each array has 100 atoms or less along a longest dimensionthereof.

In another general embodiment, a system, such as a magnetic data storagesystem or memory device, may include an antiferromagnetic nanostructureand/or atomic-scale structure as recited above; and at least one devicefor altering and/or reading the magnetic state of each of the arrays.

A method of reading and/or writing data to an atomic-scaleantiferromagnet or antiferromagnetic nanostructure such as one of thoserecited above includes detecting a magnetic state of one atom of aselected one or more arrays of the antiferromagnetic nanostructure oratomic-scale antiferromagnet. During a write operation, the orientationof the magnetic moment of atoms of the atomic-scale antiferromagnet maybe reversed for changing the magnetic state of the atomic-scaleantiferromagnet. A tunneling magnetoresistance device may be used fordetecting the magnetic state of the one atom of the atomic-scaleantiferromagnet and/or changing the magnetic state.

Preferred embodiments of the present invention include magneticnanostructures having a small number of antiferromagnetically coupledmagnetic atoms arranged on a surface in such a way that the stablestorage of magnetic information is possible. The magnetic atoms arecoupled antiferromagnetically, meaning that the magnetic moments onneighboring magnetic atoms point in opposite directions. By “stable,”“stable storage,” and “useful period of time,” what is meant is that thestate of the array of magnetic atoms (where the state may correspond tomagnetic information) is stored long enough to be useful for any type ofprocess, such as data storage or data processing. For example, thatstorage may be mere picoseconds (ps), nanoseconds (ns), or milliseconds(ms), such as at least 1 ps, at least 1 ns, at least 1 microsecond, atleast 1 ms, between 5 ps and 1000 ins, greater than 100 ms, at least 1second, at least 1 minute, at least 1 hour, at least 1 day, between 1 psand 1 hour, or any reasonable value in these ranges. Moreover, theperiod of stable storage may be represented in terms of a number ofclock cycles of a processor interacting with data stored on theinventive structures, such as 1 clock cycle or more.

An antiferromagnetic array of magnetic atoms has advantages over themore traditional ferromagnetic pattern, as the anti ferromagnetic arrayhas no long-range magnetic field associated with it. Particularly, amagnetic nanostructure having antiferromagnetically coupled magneticatoms does not have a long-range external (to the array) magnetic fieldsince the magnetic fields (of the constituent magnetic atoms) fully ormostly cancel each other. For example, anti ferromagnetic arrayscorresponding to data bits can be placed closer to each other thanferromagnetic bits without interfering with each other. This phenomenonallows for a very dense packing of these magnetic nanostructures withthe resulting advantage of increased storage density. When thesemagnetic nanostructures are used with a device such as an atomic-scaletunneling magnetoresistive device, the magnetic information can bewritten and read back with conventional electronic circuitry.

One preferred embodiment includes an array of magnetic atoms on asubstrate. The magnetic atoms are arranged in such a way that theirprimary interaction with their nearest neighbors is antiferromagnetic,i.e., the magnetic moment of neighboring atoms points in oppositedirections. The magnetic nanostructure can be a chain or atwo-dimensional structure, or it can extend into the third dimension byadding magnetic layers on top of the initial two-dimensional layer whilemaintaining antiferromagnetic order.

The magnetic atoms may be of any magnetic material. Illustrativemagnetic materials include iron, manganese, and cobalt.

The substrate may be any suitable material known in the art that doesnot destroy the antiferromagnetic character of the antiferromagneticarray. Preferably, the substrate is nonmagnetic. Illustrative materialsfor the substrate include Cu₂N and CuO. Factors to consider whenselecting a substrate material may include how the substrate affects thespacing of the magnetic atoms, and how the substrate material affectsaffinity of the atoms to become antiferromagnetically coupled.

FIGS. 1A-1C depict an array of magnetic atoms on a crystallinesubstrate. Referring to FIG. 1A, the array 100 of magnetic atoms 102 isarranged in a linear chain. Particularly, a chain of several Fe atoms isassembled on a crystalline substrate by employing established techniquesof atom manipulation on a substrate. Exemplary formation techniques arepresented below. The substrate used in this example is a monolayer Cu₂Nfilm grown on a Cu(001) surface. This substrate is used in this examplebecause it gives the Fe atoms easy-axis magnetic anisotropy, a usefulproperty for implementing stable magnetic bits because it guides thespins to point along the axis. The Fe atoms are coupledantiferromagnetically. A linear chain of atoms, such as that shown inFIG. 1A, is a simple example of a type of magnetic bit. As shown, themagnetic moment orientations of the magnetic atoms in the arrayalternate between adjacent ones of the magnetic atoms. Note that whilethis example has six magnetic atoms, various embodiments may have as fewas two magnetic atoms and up to several dozen or several hundreds ofatoms.

A chain of 8 Fe atoms 102 assembled on this surface with the spacingshown in FIGS. 1A-1C has two stable magnetic states at sufficiently lowtemperature, the so-called Néel states. The temperature for thesedemonstrations is 1 Kelvin. In a Néel state neighboring magnetic atomsare magnetized in opposing directions. This is illustrated in FIGS. 1Band 1C. In FIG. 1B, the antiferromagnetic coupling between Fe atoms 102in the chain results in one logic state, corresponding to the first Néelstate, in which the topmost Fe atom 102 in the chain has a magneticmoment that is oriented “down”. FIG. 1C depicts a second logic state ofthe bit, corresponding to the second Néel state, in which magneticmoment of the same Fe atom 102 in the chain is “up”. The chain may beany length, and longer chains tend to result in magnetic states that aremore stable. Note also the absence of any external stabilizingstructure, such as a ferromagnet.

The magnetic moment of atoms is generally due, in part, to the magneticmoment of the atom's electron spins, and in part to the magnetic momentderived from the orbital angular momentum of the atom's electrons. Theenergy of magnetic interaction between the atoms can be due to directexchange interaction, to indirect exchange interaction, and/or tomagnetic dipolar interaction. The details of these interactions areknown to those skilled in the art and discussion thereof is omitted forclarity.

A central finding is that, even though these magnetic nanostructures areof atomic-scale dimensions, they have two stable magnetic states atsufficiently low temperature. Furthermore, the antiferromagnetic arraycan be switched between these two states at will. This magneticstability and the ability to switch between the magnetic statesconstitute two of the most important elements of magnetic data storage,and are here demonstrated on the atomic scale.

FIG. 2 is representation of a scanning tunneling microscope (STM)topographic image of a chain 200 of 8 Fe atoms 202. The atoms 202 arearranged on the surface of Cu₂N as shown in FIG. 1. In this embodiment,the spacing between two neighboring Fe atoms is 0.72 nm giving the chaina length of about 6 nm. The other protrusions 203 around the chain arealso Fe atoms in different spatial arrangements. The atoms of the chain200 appear identical because the tip of the STM is not spin polarized.However, upon spin polarizing the tip, the magnetic state of one or moreof the atoms is discernible.

One method to read and write such a magnetic bit is to couple the bit toa magnetic tunnel junction. To form such a junction in one approach, acontrol electrode is placed near the antiferromagnetic nanostructure sothat electrons can flow between the control electrode and thenanostructure by means of quantum mechanical tunneling. Thenanostructure and the control electrode may be separated by aninsulating layer or a vacuum gap. The control electrode is spinpolarized, meaning that it preferentially conducts electrons of one spinorientation more than the other orientation. The resulting tunnelingjunction is then magnetoresistive, i.e., the tunneling current dependson the relative alignment of the control electrode's and nearestmagnetic atom's directions of spin polarization. This allows the stateof the array to be sensed by measuring the amount of current flowingthrough the tunnel junction.

In one approach, the magnetic tunnel junction is aligned to a selectedatom or group of atoms in the array. Reading the state of the array isperformed by measuring the conductance of the tunnel junction. Writingmay be performed by applying an appropriate voltage or current to thetunnel junction, which reverses the magnetic state of the array.

In one illustrative approach, the tip of a STM is used as the controlelectrode to perform the reading and/or writing of the magnetic bits.The tip of the STM can be made to be spin-polarized by any of severaltechniques, such as coating the tip with a ferromagnetic orantiferromagnetic material or placing a magnetic particle on the apex ofthe tip. For the present example, assume the STM tip is made to bespin-polarized by placing a magnetic atom at its apex and applying anexternal magnetic field to polarize that atom.

Due to the very high spatial resolution of the STM, the magnetic stateof such an antiferromagnetic array may be written and read, and thusutilized for data storage at very high storage density, presentlyestimated to be on the order of 100 Terabit per square inch, or about100 times denser than present commercial hard disk drive storagedevices.

FIG. 3 shows an implementation of the reading operation of an AFMmagnetic bit 302 with a spin-polarized STM tip 304. When the STM tip isplaced over a magnetic atom with its magnetic moment opposite to thespin polarization of the tip, the resulting current is low (i.e., thedevice is in a low-current tunneling magnetoresistance (TMR) state). Onthe other hand, when the tip is placed over a magnetic atom with itsmagnetic moment aligned with the tip, the resulting current is higher.Thus, when the tip 304 is placed above the leftmost atom 306 in themagnetic chain (as shown) the tunneling current is small because thespin of the tip (up arrow) points in a direction opposite that of themagnetic moment of the atom below it (down arrow).

Thus, a read operation is performed by measuring the tunnel current.Positioning the tip over the next magnetic atom to the right also readsthe nanostructure's state, but the current magnitudes are reversed. Bychoosing proper magnitudes of tunneling voltage and current the same tipcan be used to write the magnetic nanostructures into either of the twostable magnetic states, which function as the two logic states of abinary memory device.

This contrast mechanism is easily able to distinguish between the twodifferent Néel states of the bit. By holding the tip stationary overjust one of the atoms and measuring the conductance of the junction, themagnetic state of the structure can be determined. For example, a smallvoltage can be applied between the tip and the substrate and themeasured tunnel current indicates the state of the bit.

Other techniques and devices for reading and/or writing may be used. Oneexample is inclusion of a magnetic tunnel junction sensor near aterminal atom of each array.

FIGS. 4A and 4B depict another implementation of an embodiment of thepresent invention. Particularly, FIGS. 4A and 4B respectively show arepresentation of both stable states (the Néel states) of the array. Inthis type of antiferromagnetic bit, two rows of magnetic atoms 402 arecoupled together into a 2×6 array of atoms to form a single bit. Eachcolumn of six antiferromagnetically coupled Fe atoms is thenantiferromagnetically coupled to a second column of Fe atoms to form thebit. This type of array also shows the basic property of having twostates that are stable, so the magnetic state of this array structurecan be used for data storage.

Similar to the linear chain of Fe atoms discussed in FIG. 1, the regularAFM pattern in the 2×6 array results in a canceling of the dipolemagnetic fields due to the magnetic moments of the constituent Fe atoms,and therefore the 2×6 magnetic bit has no or substantially no magneticfield external to the array. Furthermore, this canceling of the magneticmoments results in the state of the magnetic bit being very insensitiveto applied magnetic fields. An experiment mimicking this arrangement wastested and observed to be stable in a magnetic field of 3 Tesla.

The structure shown in FIGS. 4A and 4B can be used at highertemperatures than the six-atom structure shown in FIG. 1 due to the factthat the two columns couple antiferromagnetically to each other, whichstabilizes the magnetic state.

The switching operation of a magnetic bit is described in FIG. 5. Theleft panel is the initial magnetic state of the bit, labeled as logic 1.After imaging the magnetic state the STM tip is positioned near one atomof the magnetic bit. A short pulse of less than 1V and a few nanosecondsduration is sufficient to switch the orientation of the magnetic momentsof all the atoms of the magnetic bit, thereby switching the bit from theinitial to the final magnetic state. The final magnetic state is thenimaged again with the same magnetic tip and is labeled as logic 0. Thisswitching is can be repeated many times without disturbing the physicalstructure of the magnetic bit.

FIGS. 5A, 5B and 5C depict reading and switching the state of anantiferromagnetic magnetic bit 500. The bit is assembled from 12 Featoms by using the tip of an STM 502 to position the atoms. The atomsappear alternately slightly higher and lower in the topograph (FIG. 5B)because the spin of the STM tip is aligned either the same or oppositeto the orientation of the magnetic moment of each atom in thenanostructure. FIG. 5A shows a measurement of the magnetic momentorientations of each atom for the initial magnetic state, labeled aslogic 1. Magnetic-moment-up atoms appear darker whilemagnetic-moment-down atoms appear lighter. Then the STM tip is broughtinto interaction with one of the atoms of the bit (FIG. 5B). Under theapplication of a short voltage pulse the magnetic state switches to thesecond stable state, labeled 0, as depicted in FIG. 5C.

An antiferromagnetic magnetic bit has several significant advantagesover the more common ferromagnetic magnetic bit. FIG. 6 shows densepacking that is possible due to the antiferromagnetic nature of themagnetic bits. Thus, some embodiments may include multiple arrays of atleast two antiferromagnetically coupled magnetic atoms present on acommon substrate, each of the arrays having at least two magneticstates, wherein each array has a net magnetic moment (or net “spin”) ofabout zero. Moreover, in particularly preferred embodiments, the arraysare positioned relative to each other such that the magnetic interactionenergy between arrays is at least partially canceled. “Partiallycancelled” can be defined in terms of a cancellation of magneticinteraction energy with an array and its nearest neighbor array, wherethe partial cancellation is 50% or more, preferably 75% or more, andmore preferably 90% or more. In some illustrative approaches, thecancellation is 50-90%, 60-80%, and 75-100%. In an ideal case, themagnetic interaction energy is 100% or about 100% canceled.

Referring again to FIG. 6, three magnetic bits 602, 604, 606 having 12Fe atoms each are arranged on a substrate of Cu₂N, and are separated bya short distance, e.g., 0.5-5 nm. In the approach shown, the distance ofseparation is 1 nm, which is just one more atomic lattice distance thanthe Fe atoms within the magnetic bit itself. As discussed above, theantiferromagnetic magnetic bits have no long-range magnetic fields andas such the long-range interactions can be expected to be negligible.However, over the short distances between the closest atoms inneighboring bits, a superexchange interaction has to be considered. Akey observation is that the net coupling of one atom to its neighbors inan adjacent magnetic bit is zero since that atom interacts equally witha magnetic-moment-up atom and a magnetic-moment-down atom of aneighboring bit, independent of the neighboring bit's magnetic state.This symmetry effectively cancels the interactions on the short length,giving rise to essentially no net interaction between neighboring,thereby allowing the high packing density while also making the bitsinsensitive to applied magnetic fields.

FIG. 7 depicts an embodiment of two magnetic bits 702, 704 on a commonsubstrate of Cu₂N. Particularly, two magnetic bits are shown, each madefrom 12 Fe atoms on a substrate of Cu₂N. The depiction of the substrateis 45 degrees rotated in comparison to the orientation of the substratein FIG. 6. However, the successive bits are placed on a 45 degree anglewith respect to the antiferromagnetic chains within the bits. Densepacking of magnetic bits 702, 704 is again possible since the Fe atomsinside each bit are coupled almost equally to the Fe atoms of oppositemagnetic moment orientation in the neighboring bit. This embodimentdemonstrates that the exact arrangement of atoms is not critical as longas a correct balancing of the short length scale magnetic interactionenergy is fulfilled.

FIG. 8 shows a larger assembly 800 of AFM magnetic bits. The depictionshows eight magnetic bits arranged in the packing arrangement asrepresented in FIG. 7. Each individual bit is composed of a regulararray of 2×6 Fe atoms. In total, the byte includes 96 preciselypositioned Fe atoms. This magnetic byte demonstrates data storage on theatomic scale at a storage density or about 100 Terabits per square inch.

The requirement of atomic-scale precision when aligning the controlelectrode to the magnetic bit can be relaxed in some cases, such as whenthe control electrode is an antiferromagnet with similar crystal latticedimensions as the bit, or when several atoms of the antiferromagneticbit that have the same magnetic moment alignment are used to form thetunnel junction. For example, the top-most two atoms in each bit of FIG.7 have the same magnetic moment alignment, as does the top plane ofatoms in FIG. 9B.

The previous description has primarily been directed to 1- and2-dimensional arrays of magnetic atoms. Storing magnetic information inan AFM structure can also be performed using 3-dimensional structures,in some embodiments. Exemplary 3-dimensional configurations 900, 950 areschematically shown in FIGS. 9A and 9B, respectively.

In one embodiment, an antiferromagnetic nanostructure includes one ormore arrays, where each array includes at least two layers of themagnetic atoms in a stacked configuration, the atoms in each layer beingantiferromagnetically coupled to other atoms in the same layer, where anet magnetic moment of each layer is about zero. In the exemplaryembodiment shown in FIG. 9A, an AFM layer is positioned on top ofanother AFM layer, and the AFM order is thus continued into the thirddimension. The coupling between atoms within a horizontal layer isantiferromagnetic, as illustrated by the arrows showing the orientationof the magnetic moments, so the net magnetic moment of the entirestructure is about zero. Coupling between and within successive layersmay be antiferromagnetic, ferromagnetic, or both, depending on thedetails of the crystal structure, as long as it maintains alternation inmagnetic moment orientation along some dimension.

In one embodiment, an antiferromagnetic nanostructure includes one ormore arrays, where each array includes at least two layers of themagnetic atoms in a stacked configuration, the atoms in each layer beingferromagnetically coupled to other atoms in the same layer (i.e., havingthe same magnetic moment orientations as the other atoms in the samelayer), wherein a net magnetic moment of adjacent layers is about zero.In the structure of FIG. 9B, for example, a ferromagnetic layer is puton top of another ferromagnetic layer, and successive layers haveopposite magnetic moment orientation. On the right the magnetic momentswithin any horizontal layer are aligned (ferromagnetic ordering) but thecoupling between layers is antiferromagnetic, so the net magnetic momentis again essentially zero.

Antiferromagnetic order is thus achieved in the third dimension. Athree-dimensional embodiment has the advantage of a higher packingdensity for the same number of magnetic atoms in a magnetic bit. Becausethe thermal stability is expected to grow with the number of magneticatoms in the bit, an extension into the third spatial dimensionconsequently enables extreme areal data density with good performance.

Many techniques for fabricating the antiferromagnetic arrays arepossible. In one approach, a technique employing self-assembly ofmagnetic atom patterns on surfaces may be used. Another approach employssynthesis of antiferromagnetic molecules that contain the magnetic atomsand couples them in an antiferromagnetic arrangement.

In yet another approach, the tip of an STM is used to arrange the atomson the substrate, to construct the magnetic nanostructure. The same oranother STM may be used to perform the reading and writing of themagnetic bits. An STM inherently has atomic-scale spatial resolution andas such allows the direct demonstration of various embodiments.

In one illustrative embodiment, depicted in FIG. 10, an STM 1000 is usedto place Fe atoms 1002 in a regular pattern on a substrate, e.g., ofCu₂N. The Fe atoms may each be placed at a binding site on the substratein which they have a large magnetic anisotropy, causing the Fe atom'smagnetic moment to align parallel to the resulting easy axis D. Thisanisotropy impedes the magnetic moment from switching between the twopossible orientations. Neighboring Fe atoms interactantiferromagnetically with exchange coupling energy J. This anisotropyand exchange coupling may allow the magnetic moments to be describedusing the Ising model, in which the magnetic moments always point alongone axis and quantum superposition states may not have to be considered.

With continued reference to FIG. 10, the AFM array can be switchedbetween the two Néel states by tunneling electrons from the STM tip1000. To switch the magnetic state, an STM tip is held stationary overany Fe atom of the structure and tunnel current passed through it untila step is observed in the current. See, e.g., FIG. 11A.

Experimental

This section discusses experimental results. The following descriptionis not meant to be considered limiting on the present invention in anyway. Rather, the following description is provided by way of exampleonly.

Sample Preparation

All experiments were performed in a low-temperature STM equipped with avariable magnetic field. Cu(100) single crystals were cleaned byrepeated sputter-anneal cycles. One monolayer of Cu₂N was formed bynitrogen ion bombardment of the clean Cu(100) near room temperature, andsubsequent annealing to about 300° C. Fe (and Mn) atoms were depositedonto the cold sample surface at 4.2 K with a density of ˜1% of amonolayer.

Fe atoms were positioned 0.72 nm apart on the two-fold symmetric Cubinding sites of the Cu₂N overlayer using vertical atom manipulation.Surface and spacing were chosen to give magnetic coupling that isadequate to demonstrate antiferromagnetism, while keeping the atoms wellenough separated to clearly resolve the location and magnetic momentorientation of each one. For atom pick-up, the probe tip was loweredclose to point-contact (˜100 kOhm junction resistance) and a samplevoltage of +1.7 V was applied while withdrawing the tip. Drop-offemployed a two-step process in which the atom was first positioned atopa nitrogen surface atom (N-binding site) by lowering the loaded tip intopoint-contact and withdrawing at zero voltage, and subsequently thedropped atom was hopped laterally to a Cu binding site with a +0.75 Vvoltage pulse with the tip positioned laterally to guide the atom to theintended binding site.

Spin Polarized Tips

Spin-polarized tips were created by transferring one or more magneticatoms (Fe or Mn) from the surface to the apex of the tip. Such a tipbehaves like a paramagnet and gives spin-polarized tunnel currents at<10K when external magnetic fields of >0.5 T are applied. The degree ofspin-polarization of the tips was determined from measurements onisolated Fe and Mn atoms. Using the customary definition of thespin-polarization, η, we determined η=0.6 for the tip used in FIG. 10and η=0.3 for the tip in FIG. 13C. Tips with reverse spin-polarization(anti-aligned to the magnetic field) were produced by placing more thanone magnetic atom at the apex, the atoms presumably coupling to eachother antiferromagnetically. The magnetic contrast does not dependsignificantly on the magnitude or sign of the applied voltage as long asthe junction voltage is small enough (<˜5 mV) to avoid disrupting theorientations of the magnetic moments of the AFM structure.

Array Magnetic State Switching

In experiments in which the magnetic states were switched, it wasobserved that the state switches most readily when the tip was placedover an atom at the end of an array. For any voltage and currentapplied, the switching between magnetic states was found to occur with auniform probability per unit time, which can be characterized by way ofa switching rate. This rate increased rapidly when the tunneling currentwas increased by moving the tip closer to the surface, as discussedbelow with reference to FIG. 11C.

In one experiment, switching between Néel states was induced bytunneling electrons. FIG. 11A is a chart depicting tunnel current as afunction of time with an STM tip placed over an end atom of a 1×8 arrayof Fe atoms at 7 mV. The chain switches its magnetic state about twiceper second. Referring to FIG. 11B, the same technique was applied,except that shorter voltage pulses were applied to demonstrate fastswitching. Particularly, pulses of 500 mV and 10 ns duration wereapplied every 10 ms. Between pulses only 2 mV, which was below thethreshold for switching, was applied to sense the magnetic state.Referring to FIG. 1 IC, the switching rates from magnetic state ‘0’ to‘1’ (dots 1110) and ‘1’ to ‘0’ (dots 1112) at 7 mV and different tunnelcurrents, were obtained by changing the tip-sample distance. Theswitching rates increased according to a power law dependence withexponent k=1.1 (‘0’ to ‘1’) and 1.9 (‘1’ to ‘0’). FIG. 11D is a chartgraphically depicting switching rates at different voltages for the samearray from which the data was obtained for FIG. 11C. As shown, fasterswitching was obtained at higher voltages. Voltage was appliedcontinuously (for V<10 mV), and as 5-1000 ns pulses (for V>10 mV). Themagnetic field applied was 1 T for all panels (FIGS. 11A-11D), thetip-sample distance was set at 20 pA, and the voltage was 2 mV for(FIGS. 11A-11C).

With the tip at a fixed height, the switching rate increased abruptlynear the threshold voltage and quickly exceeded the bandwidth of theSTM's current amplifier (FIG. 11B). A pulsed excitation scheme was usedto obtain a quantitative measure of the switching rate in this regime.Sub-microsecond pulses were applied and each was followed by a 100 mslow-voltage window in which the resulting state was monitored.

Switching rates were extracted from the measured probabilities forswitching at each pulse. The switching rate increased faster than inproportion to the voltage up the highest voltage tested, with switchingtimes of ˜20 ns at 0.5 V and ˜5 nA (FIG. 11D). This demonstrateselectrical switching of the AFM nanostructures at high speeds andfemtojoule energies.

The AFM ordering can be extended in two dimensions to form arrays. FIG.12A depicts a (2×6) and (2×4) array of Fe atoms. These arrays each havetwo chains of the kind shown in FIG. 10, with the two chains coupled toeach other antiferromagnetically with a coupling J′=0.035 meV betweeneach pair of atoms, as represented in FIG. 12B, which is a schematic ofthe atomic positions of Fe and Cu₂N substrate atoms in (2×n) and (1×n)arrays. This inter-chain coupling is much weaker than the couplingwithin each chain even though the spacing between the atoms isidentical. This difference is believed to be due to the structure of thebinding site of the Fe atom. Despite the weak coupling between thechains, the arrays show stable antiferromagnetic order with much greaterstability than either chain alone.

To investigate the magnetic stability of the Néel states, the thermalswitching rates of the arrays were examined. At 1.2 K both the (2×6) andthe (2×4) arrays are stable in either Néel state. In contrast, at 5.0 Kboth nanostructures switch spontaneously between their two Néel states.At an intermediate temperature of 3.0 K only the (2×6) array is stabledemonstrating that the blocking temperature—where magnetic structureslose their permanent spin state—increases with the number of atoms inthe array.

A quantitative study of the thermal stability of the Néel states (FIG.12C) shows that each structure follows an Arrhenius law over theobserved range of switching rates. From this an effective energy barrierand pre-exponential factor can be deduced for each structure, as shownin Table 1, which lists Arrhenius parameters for thermal switching ratesof FIG. 12C. The Arrhenius function τ−τ₀ exp(E_(B)/k_(B)T) is fitted tothe temperature-dependent residence times of each structure.

TABLE 1 Energy barrier Array size Magnetic field Prefactor τ₀ (s) E_(B)(meV) (2 × 6) 1 T, 3 T 3 × 10⁹ 8.2 ± 0.2 (1 × 8) 1 T 5 × 10⁹ 6.9 ± 0.1(1 × 8) 3 T 5 × 10⁸ 5.6 ± 0.3 (2 × 4) 1 T, 3 T 2 × 10⁴ 1.49 ± 0.03

The two Néel states occur with equal frequency, as expected fordegenerate states, and the switching events occur with a fixedprobability per unit time. This is consistent with a model in whichtransitions between the Néel states require excitation over a spinreversal barrier. The energy barriers and prefactors are only weaklysensitive to magnetic field, which shows that the behavior of thestructures is essentially unchanged over a wide range of appliedmagnetic fields.

For the (2×6) and (1×8) structures, the switching barriers obtained fromthe Arrhenius fit were ˜6-8 meV (Table 1), which are comparable to theenergy 2S²J=9.6 meV needed to create a single Ising domain wall withinone of the chains by flipping the orientation of the magnetic moment ofone or more consecutive atoms at the end of a chain. Here S=2 is themagnitude of the spin of the Fe atom. This energy barrier is alsocomparable to the threshold voltage for current-induced switching (FIG.2). This suggests a switching process that reverses one chain at a timeby propagating a domain wall along each chain. The (2×6) array is highlystable at low temperatures; we experimentally determined a lower limitfor the stability to less than 1 switching event per 17 hours at 0.5 K.

In contrast, the (2×4) structure has a much smaller barrier, only 1.4meV, which is comparable to 4×2S²J=1.1 meV, the energy required tofrustrate the weak coupling between the two chains but not enough tocreate a domain wall within a chain. Together with the much reducedprefactor, this low barrier points to a reversal process in which oneentire chain switches in a thermally-assisted magnetic tunnelingprocess. Such tunneling of magnetization is often observed in molecularmagnets.

To achieve long-term stability, a switching barrier of some 50 k_(B)T(for antiferromagnets just as for ferromagnets) is desirable, which is1.3 eV at room temperature.

This is about 100 times higher than the presently-described barrier,which is small due to the ease of introducing a domain wall, aconsequence of small J. Much stronger coupling may be readily obtainedon this surface and in typical AFMs by placing the atoms closertogether. Using such stronger coupling, and the anisotropy barrieralready present for Fe on this surface, room temperature stability maybe achieved with ˜150 atoms.

A major obstacle for traditional magnetic storage media is theinteraction of neighboring bits due to their net magnetic moments andresultant dipolar magnetic fields. However, at atomic dimensions,exchange interactions can still cause undesired coupling between bits.The following description demonstrates how antiferromagnetic orderwithin each bit can be used to compensate even these short rangeinteractions. FIG. 13A shows a dense array of eight independent AFM bitsthat are spaced 0.9 nm from each other. The short-range decoupling isachieved through magnetic frustration. A staggered assembly that placesthe atoms of any given bit symmetrically between the atoms of theneighboring bits results in near-perfect cancellation of inter-bitcouplings (FIG. 13B). In essence, an atom in a given bit couples equallyto two atoms with opposing magnetite moment orientations in theneighboring bit (J_(b) in FIG. 13B), which effectively cancels thesepairs of magnetic interaction energies.

The 8 AFM bits can store one byte of magnetic information, e.g., each ofthe eight 12-atom arrays can be switched between its two Néel statesindependent of the others. FIG. 13C depicts short sequences of testarrangements written into the byte. Particularly, in the top image ofFIG. 13C, all eight bits are in a ‘1’ state. In the middle image, analternating pattern of ‘1’ and ‘0’ is depicted. In the bottom image ofFIG. 13C, all bits are in a ‘0’ state.

Each configuration is stable over hours and read-out was easily achievedby topographic imaging. Each bit occupies an area of only 9 nm²,including a spacer area, resulting in a net areal data density of about70 Tbits/inch².

The arrangement of Fe atoms that form each bit in the byte is a varianton the (2×6) array (compare FIG. 12B and FIG. 13B) in which the ends ofeach bit are beveled to give the end-most atoms of each bit the samemagnetic moment orientation, which gives visual clarity in viewing thestate, and illustrates that the exact arrangement of atoms is notcritical for stability.

This work demonstrates that switchable nanoscale antiferromagnets arecandidates for memory, storage, and spintronic applications. The desirefor atomically precise alignment in a read or write electrode may berelaxed by terminating the AFM array such that all magnetic moments inone face point in the same direction.

It will be clear that the various features of the foregoingmethodologies may be combined in any way, creating a plurality ofcombinations from the descriptions presented above.

While various embodiments have been described above, it should beunderstood that they have been presented by way of example only, and notlimitation. Thus, the breadth and scope of an embodiment of the presentinvention should not be limited by any of the above-described exemplaryembodiments, but should be defined only in accordance with the followingclaims and their equivalents.

What is claimed is:
 1. An anti ferromagnetic nanostructure, comprising:an array of at least two antiferromagnetically coupled magnetic atomshaving at least two magnetic states that are stable for at least onepicosecond even in the absence of interaction with an externalstructure, the array having a net magnetic moment of zero or about zero,wherein the array has 100 atoms or less along a longest dimensionthereof.
 2. An antiferromagnetic nanostructure as recited in claim 1,wherein magnetic moment orientations of the magnetic atoms in the arrayalternate between adjacent ones of the magnetic atoms.
 3. Anantiferromagnetic nanostructure as recited in claim 1, wherein the arrayincludes at least two layers of the magnetic atoms in a stackedconfiguration, the atoms in each layer being antiferromagneticallycoupled to other atoms in the same layer, wherein a net magnetic momentof each layer is about zero.
 4. An antiferromagnetic nanostructure asrecited in claim 1, wherein the array includes at least two layers ofthe magnetic atoms in a stacked configuration, the atoms in each layerbeing ferromagnetically coupled to other atoms in the same layer,wherein a net magnetic moment of adjacent layers is about zero.
 5. Anantiferromagnetic nanostructure as recited in claim 1, wherein multiplearrays of at least two anti ferromagnetically coupled magnetic atoms arepresent on a common substrate, each of the arrays having at least twomagnetic states, wherein each array has a net magnetic moment of aboutzero.
 6. An antiferromagnetic nanostructure as recited in claim 5,wherein the arrays are positioned relative to each other such that amagnetic interaction energy between adjacent arrays is at leastpartially canceled.
 7. An antiferromagnetic nanostructure as recited inclaim 5, wherein the arrays are positioned relative to each other suchthat a magnetic interaction energy between adjacent arrays is canceledby at least 50%.
 8. A system, comprising: an antiferromagneticnanostructure as recited in claim 5; and at least one device foraltering and/or reading the magnetic state of each of the arrays.
 9. Asystem as recited in claim 8, wherein the at least one device is atunneling magnetoresistance device.
 10. An antiferromagneticnanostructure as recited in claim 1, wherein the at least two magneticstates are stable for at least one microsecond.
 11. An antiferromagneticnanostructure as recited in claim 1, wherein the at least two magneticstates are stable for at least one second.
 12. An antiferromagneticnanostructure as recited in claim 1, wherein the at least two magneticstates are stable for at least one hour.
 13. An atomic-scale structure,having a net magnetic moment of zero or about zero; two or more stablemagnetic states; and having an array of atoms that has magnetic momentsthat alternate between adjacent magnetic atoms along one or moredirections.
 14. An atomic-scale structure as recited in claim 13,wherein the array includes at least two layers of the magnetic atoms ina stacked configuration, the atoms in each layer beingantiferromagnetically coupled to other atoms in the same layer, whereina net magnetic moment of each layer is about zero.
 15. An atomic-scalestructure as recited in claim 13, wherein the array includes at leasttwo layers of the magnetic atoms in a stacked configuration, the atomsin each layer being ferromagnetically coupled to other atoms in the samelayer, wherein a net magnetic moment of adjacent layers is about zero.16. An atomic-scale structure as recited in claim 13, wherein multiplearrays of at least two antiferromagnetically coupled magnetic atoms arepresent on a common substrate, each of the arrays having at least twomagnetic states, wherein each array has a net magnetic moment of aboutzero.
 17. An atomic-scale structure as recited in claim 13, wherein thearrays are positioned relative to each other such that a magneticinteraction energy between adjacent arrays is at least partiallycanceled.
 18. An atomic-scale structure as recited in claim 13, whereinthe two or more magnetic states are stable for at least one second. 19.A system, comprising: an atomic-scale structure as recited in claim 13;and at least one device for altering and/or reading the magnetic stateof each of the arrays.
 20. An antiferromagnetic nanostructure,comprising: multiple arrays each corresponding to a bit, each arrayhaving at least eight antiferromagnetically coupled magnetic atoms, eacharray having at least two readable magnetic states that are stable forat least one picosecond, each array having a net magnetic moment of zeroor about zero, wherein no external stabilizing structure exertsinfluence over the arrays for stabilizing the arrays, wherein each arrayhas 100 atoms or less along a longest dimension thereof.
 21. A system,comprising: an atomic-scale antiferromagnet as recited in claim 20; andat least one device for altering and/or reading the magnetic state ofeach of the arrays.
 22. A method of reading and/or writing data to theantiferromagnetic nanostructure of claim 20, the method comprising:detecting a magnetic state of one atom of a selected one or more arraysof the antiferromagnetic nanostructure.
 23. A method as recited in claim22, further comprising reversing magnetic moment orientations of atomsof the atomic-scale antiferromagnet for changing the magnetic state ofthe atomic-scale antiferromagnet.
 24. A method as recited in claim 22,further comprising using a tunneling magnetoresistance device fordetecting the magnetic state of the one atom of the atomic-scaleantiferromagnet.